Crosslinked, polymeric oxygen barrier coatings for containers, known in the art, require long thermal curing and are sensitive to humidity.
PCT patent application WO 00/49072, for example, describes a coating system for films and rigid containers, especially for injection stretch blow molded poly(ethylene terephthalate) (PET) bottles, which dramatically improves gas barrier performance over uncoated PET. The gas barrier coating layer is formed from a water-borne composition which contains at least 2% by weight solids. The solids portion of the water-borne barrier composition contains generally from 10% up to less than 90% by wt. of an inorganic laminar mineral and from greater than 10% by wt. up to less than 90% by wt. of a water-soluble or water-dispersible film forming organic binder. This binder optionally contains a catalyst and one or more crosslinking agents. While the disclosed coating provides enhanced barrier to gas transport, the time required to cure the binder thermally is long. The barrier coating is also sensitive to moisture, such that barrier properties are degraded at high humidity, above 50% relative humidity. Thus, both a rapid binder curing process and lessened coating moisture sensitivity are needed for practical application.
Patent application JP2789705 describes a barrier coating formulation for biaxially oriented films. It includes a silyl-group-containing modified poly(vinyl alcohol) and synthetic hectorite at a 50:50 weight ratio, dried and then heat-treated at 130 to 150° C. This cure temperature is too high for PET bottles to maintain their shape.
U.S. Pat. No. 5,700,560 discloses a coating formulation containing platy clay and polymer, as well as zirconium catalyst. A curing temperature above 100° C. precludes this coating from being suitable for PET bottles.
PCT patent application WO 98/56861 discloses a coating formulation of platy clay and water soluble polymers, such as poly(vinyl alcohol), with glucose as a plasticizer. The oxygen transmission rate (OTR) of poly(vinyl alcohol) is known to degrade at high humidity (see, e.g., U.S. Pat. No. 3,516,960), and there is no indication that these coatings are any less moisture sensitive.
European Patent Application 805,177 discloses a coating formulation, for biaxially oriented films, comprising platy clay, water-soluble polymer, and an amine compound. Curing is done above 95° C., making its use for PET bottles doubtful.
U.S. Pat. No. 5,840,825 discloses a coating formulation comprising platy clay and polymers, specifically, polyamine and polyepoxide. Curing is done at 63° C. for thirty minutes, which is a long time for an economically viable, high volume process.
Japanese Patent Application 11-182306 discloses a gas barrier film coated with a formulation comprising an inorganic layer-like compound (for example, montmorillonite clay), polyvinyl alcohol, and boric acid. The addition of boric acid resulted in a barrier improvement of about 2 to 3×. After samples were stored at 40° C., 90% RH for a month, barrier deteriorated by about the same relative amount whether or not boric acid was present, in spite of the crosslinking of the polyvinyl alcohol by the boric acid; that is, samples with coatings containing boric acid that had been stored at 40° C., 90% RH also exhibited about 2 to 3× better barrier than samples without boric acid that had been stored at 40° C., 90% RH.
A need therefore still exists for a high barrier coating formulation for PET containers and other polymeric substrates where the coatings have barrier properties that are insensitive to humidity. There is a further need for a curing system in such a barrier formulation that can be cured rapidly, preferably at low temperature.
The present invention relates to an improved water-borne gas barrier coating composition for polymeric substrates such as films, sheet, tubing and rigid, i.e., shaped, containers, and, more particularly, to an improved organic binder system for such a barrier coating system which includes a laminar layer mineral in a gas barrier layer, wherein the organic binder system exhibits lessened sensitivity to moisture and decreased curing time relative to prior art.